Hydronium is a positively-charged ion that forms from
water in the presence of
hydrogen ions. The formula for hydronium is H
3O
+.
Hydronium is present whenever an acid is dissolved in water; the hydrogen ions released by the acid do not exist in a free state. Water will also spontaneously dissociate into hydronium and hydroxide ions, in the following reaction:
- <math>2\mbox{H}_2\mbox{O} \Leftrightarrow \mbox{H}_3\mbox{O}^+ + \mbox{OH}^-</math>
The equilibrium of this reaction is far to the left under most conditions, and so pure water will have little hydronium in it. About 1 molecule in 10,000,000 is dissociated.
Since the reaction releases an OH_ ion, this ion's alkalinity balances hydronium's acidity, and water remains neutral (a hydronium ion concentration of 1 x 10-7.0).
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